E-fuse (“electronic fuse”) circuits control a series power transistor that connects a circuit to a power supply. For example, a circuit board may get its power from a bus. When the circuit board is inserted into the bus socket, contacts in the bus socket connect the board to power. Among other functions, an e-fuse controls how the power is applied as the board is powered up. E-fuses often have the following basic controls:                a. Soft start (“dv/dt control”) during initial power up, where the voltage and current supplied to the gate of the power transistor is ramped up at a user set slope so that the output voltage is ramped slowly.        b. Current limit control, where the power transistor current is monitored and compared against a user set current limit. If the current exceeds the set limit, the e-fuse tries to limit the current by controlling the gate of the power transistor.        c. Over voltage protection: protection where the input is monitored and compared against a set reference. If the voltage exceeds the reference the gate of power transistor is pulled to zero turning off the power transistor.        
In an example application, a power transistor has a drain terminal coupled to a voltage supply and a source terminal coupled to the load. The gate of the power transistor needs to be at a sufficient voltage to turn on the power transistor to provide the power supply voltage level to the output (Vout). To turn on a typical power transistor, the gate voltage has to exceed the source voltage by a threshold voltage. Because the transistor has a low on-resistance (Rdson), the source voltage is at approximately the input voltage, so that the output voltage is approximately equal to the input voltage. Thus, to turn on the power transistor the voltage applied to the gate of the power transistor must be higher than the voltage provided by input power supply (Vin), which is usually the highest voltage level available. A charge pump is usually required to provide a higher voltage at the gate terminal of the power transistor. However, charge pumps are quite inefficient and have large current requirements. Only about 20% of the power used by a charge pump is provided at the output of the charge pump. In addition, to provide large currents, charge pumps require large capacitors, which require a very large area of an integrated circuit.